LazyBox.swift

protocol Locking {
  func lock()
  func unlock()
}

protocol Synchronizing {
  func sync<T>(execute: () throws -> T) rethrows -> T
}

extension Locking {
  func sync<T>(execute: () throws -> T) rethrows -> T {
    defer { unlock() }
    lock()
    return try execute()
  }
}

extension NSLock: Locking, Synchronizing { }

final class LazyBox<T> {
  private let compute: () -> T
  private var _value: T?
  private let _lock = NSLock()

  init(_ compute: @escaping () -> T) {
    self.compute = compute
  }

  var value: T {
    if let _value = _value { return _value }
    return _lock.sync {
      if let _value = _value { return _value }

      let v = compute()
      _value = v
      return v
    }
  }
}